(1) Field of the Invention
The present invention relates to a method for forming an isolation region for electrically isolating elements, more particularly, to a method for forming an isolation region by which a semiconductor device having a high integration density can be produced.
(2) Description of the Prior Art
It is well known that an isolation region, i.e., a region for isolating elements from each other, is formed by the steps of forming a silicon nitride (Si.sub.3 N.sub.4) layer on a silicon substrate, patterning the silicon nitride, and then forming a silicon dioxide layer by thermal oxidation of the surface of the silicon substrate using the patterned silicon nitride layer as a masking film.
In the above-mentioned steps, however, particularly in the thermal oxidation of the surface of the silicon substrate, the silicon layer located below the edge portion of the silicon nitride layer is simultaneously oxidized. The resultant silicon dioxide layer raises the edge portion of the silicon nitride layer. This silicon dioxide layer, commonly called a "bird's beak" due to its configuration, extends to the active region and therefore has a detrimental effect on the integration density.
In an improved method for forming an isolation region, grooves are formed in the surface portion of the silicon substrate by using a photolithography technique. Thermal oxidation or a chemical vapor deposition (CVD) process is then used to form an insulating layer on the grooves, thereby effecting element isolation. The smallest possible width of the grooves formed in the surface of the silicon substrate, namely, the width of the isolation region, is presently only 1 .mu.m due to limitations of the photolithography process. It has not been possible up until now to form submicron-width isolation regions.